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Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Jul. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Average price for redemption of limited partnership units				$ 11.26		$ 12.97	$ 12.89	$ 11.40
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired	5,574	696	1,993		726,539		734,802
Aggregate purchase price of Class A common stock							$ 9,806	$ 14,557
Average price of Class A common stock	$ 11.76	$ 11.85	$ 10.58		$ 13.37		$ 13.35
Class A Common Stock [Member]
Class of Stock [Line Items]
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired				8,263		295,871	734,802	1,246,365	1,386,769
Aggregate purchase price of Class A common stock				$ 100		$ 4,000	$ 9,800	$ 14,600
Average price of Class A common stock				$ 11.48		$ 13.41	$ 13.35	$ 11.68
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units				2,100,000		1,200,000	8,000,000	5,800,000
Aggregate redemption price of limited partnership units				$ 23,500		$ 16,000	$ 103,500	$ 66,100
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units				17,800		400,000	100,000	700,000
Aggregate redemption price of limited partnership units				$ 200		$ 5,000	$ 1,300	$ 8,000
Average price for redemption of founding/working partner units				$ 12.33		$ 11.48	$ 12.98	$ 11.19

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Total change in each class of partners' capital accounts during the year due to redemptions and adjustments to redemption value. All partners include general, limited and preferred partners.

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